Structural Adaptations of Plants

Structural adaptations of plants to reduce rate of transpiration

When the rate of transpiration is too high, it can have detrimental effects on the plant, as you will see in the next section on wilting and guttation. For this reason, plants have developed structural adaptations to minimise the amount of water loss.

• Position of stomata: Stomata are found on both surfaces of the leaf but there are usually more on the ventral (lower) surface of the leaf. This means that less water vapour is lost because the ventral side of the leaf is in the shade and therefore does not get as hot.

• Sunken stomata: some plants such as xerophytes have sunken stomata as a way of preventing water loss. Xerophytes (pronounced "zero-phytes") are plants that are normally found in hot, dry areas such as deserts. The sunken stomata creates a small pocket of moist air. The high humidity in the air pocket reduces the water potential gradient between the leaf air spaces and the exterior, and therefore decreases the rate of transpiration.

  

  Sunken stomata.

• Thickened cuticle: Some plants that occur in dry places have a thick cuticle that reduces transpiration.

  

  Desert plants like cactus have thick cuticles to avoid water loss.

• Hairs on leaves:Hairs trap a small layer of water vapour that works in three ways to reduce transpiration:

  • Creates a pocket of moist air to reduce the water potential gradient.
  • Increases the sheen on leaves to make them more reflective.
  • The combination of the above effects result in a cooling effect that also decreases transpiration.
  

  Hairy leaves to trap water.

• Reduction of leaf size:Small leaves have a smaller surface area for transpiration to occur.

• Leaf spines: Some plants have spines instead of leaves. Spines usually have thicker cuticles and a very small surface area, which decreases transpiration.

  

  Spiny leaves have a small surface area to decrease transpiration.

• Leaf arrangement: vertical leaf arrangement (like proteas) decrease the surface area exposed to the sun in the heat of the day, In rosette arrangements the upper leaves shield the lower leaves from the Sun.
• Rolling of leaves:When leaves roll up, water vapour gets trapped in the tunnel made by the leaf, therefore reducing the water potential gradient, and therefore reducing the rate of transpiration.